This invention is directed to a surface breakdown igniter for mercury arc devices, and is an igniter which is particularly useful for liquid metal plasma devices which are repetitively ignited.
Various types of igniter devices have been applied in prior and present day liquid metal arc devices, and particularly mercury arc rectifiers and inverters. In rectifier service conduction must be initiated whenever a forward potential is applied. Since the rectifier does not hold off voltage in the forward direction a keeper anode with a keeper discharge can be employed as long as the plasma does not extend into the anode region. Thus, in rectifier practice keeper anodes have been widely used and highly developed.
On the other hand, in inverter service the liquid metal plasma valve must hold off the voltage until the proper phase angle, and then the plasma is ignited to permit forward conduction. Since forward voltage is applied at all times, a keeper anode cannot very well be employed because the presence of plasma in the cathode region will permit forward conduction at unwanted times. The boron carbide igniter is presently the only type used in liquid metal plasma valves which operate as switches or in inverter service. This type of igniter, which operates on a different principle than the surface breakdown igniter has the following characteristics. The contact pressure between the boron carbide igniter and the liquid metal plasma valve cathode must be mechanically adjustable and therefore a mechanical linkage is required for this adjustment, and a control circuit is required to accomplish the adjustment. Furthermore, provision must be made to control the temperature of the boron carbide igniter independently of the cathode temperature. This provision also necessitates control circuitry since the required igniter temperature is dependent on the liquid metal plasma valve operating parameters. Furthermore, the boron carbide igniter is very complicated. Many more parts and consequent costs are involved in association with the boron carbide igniter. This greater complexity is principally associated with the mechanically moveable parts, and there is additional expense in connection with the controls.
On the other hand, there has been prior activity which employs some form of surface breakdown mechanism but which is not suitable for a liquid metal plasma valve ignition. This prior art includes commercially available igniters used to ignite air-fuel mixtures in jet engines, and the like.